The present invention relates to a switching regulator control circuit and a switching regulator control method, and is suitably applicable, for example, to a PWM (Pulse Width Modulation) switching regulator that automatically switches a carrier frequency in accordance with the condition of a load.
In recent years, household and industrial electronic devices have microcontrollers. A power supply system including a DC-DC (Direct Current to Direct Current) converter is widely used as a stabilized DC power supply for driving such a microcontroller. A switching regulator is mainly used as the DC-DC converter. The switching regulator includes an output power transistor and a switching regulator control circuit for controlling the on/off operation of the output power transistor. The switching regulator control circuit is mounted on a power supply IC (Integrated Circuit). The power supply system for the microcontroller is required to have high efficiency both during standby and during operation of the microcontroller. At light load such as during standby, power consumed by the switching regulator control circuit becomes larger than power consumed by the load such as the microcontroller, which reduces the efficiency of the power supply system. Therefore, it is particularly important to enhance the efficiency at light load. Among element circuits in the switching regulator control circuit, an internal oscillator and a transistor drive circuit mainly consume power. The internal oscillator generates a carrier signal. The transistor drive circuit drives the output power transistor which operates at the same frequency as the carrier signal.
Typical switching regulator control circuits include a PWM switching regulator control circuit and a PFM (Pulse Frequency Modulation) switching regulator control circuit. Japanese Unexamined Patent Publication No. Hei 11 (1999)-155281 (Patent Document 1) discloses an example of the PWM switching regulator control circuit. Japanese Unexamined Patent Publication No. Hei 11 (1999)-235023 (Patent Document 2) discloses an example of the PFM switching regulator control circuit.
The operating conditions of the switching regulator include a continuous current mode and a discontinuous current mode. The discontinuous current mode is occasionally referred to as an intermittent current mode. According to Japanese Unexamined Patent Publication No. 2006-166667 (Patent Document 3), when a synchronous rectification switching regulator goes into the discontinuous current mode in a light load condition such as standby condition or sleep mode, the efficiency decreases significantly. For this reason, Patent Document 3 discloses a switching regulator that can switch between synchronous rectification and diode rectification. In the switching regulator, an error voltage obtained by amplifying a difference between a voltage obtained by dividing an output voltage of the switching regulator and a first reference voltage is outputted, a drive pulse of the switching regulator is generated based on the error voltage, and synchronous rectification is switched to diode rectification based on a comparison between the error voltage and a second reference voltage.
According to Japanese Unexamined Patent Publication No. 2008-109761 (Patent Document 4), there is known a method for determining whether the DC-DC converter is operating in continuous current mode or discontinuous current mode to determine the magnitude of the load. The method for determining the continuous current mode or discontinuous current mode based on the voltage drop of a resistor placed in series with an inductor in the DC-DC converter causes loss due to a current flowing through the resistor and therefore reduces the power conversion efficiency of the DC-DC converter. For this reason, Patent Document 4 discloses an operation mode determination unit capable of accurately determining the operation mode of the DC-DC converter with a simple circuit configuration without reducing the power conversion efficiency of the DC-DC converter. The operation mode determination unit determines whether the DC-DC converter is operating in continuous current mode or discontinuous current mode based on the detection result of an output terminal voltage of a switching element during the off time of the switching element in the DC-DC converter. The DC-DC converter outputs a DC voltage by controlling the charging/discharging of energy in an inductor and a capacitor by the on/off operation of the switching element. In the case of a step-down DC-DC converter, the switching element is a P-channel field-effect transistor. One terminal of the inductor is coupled through the P-channel field-effect transistor to an output terminal of an input voltage, and coupled to a cathode of a diode. The other terminal of the inductor is coupled through the capacitor to an anode of the diode. The operation mode determination unit makes the determination based on a gate potential and a drain potential of the P-channel transistor.
As a solution to the problem that the efficiency of the PWM switching regulator decreases at light load, Japanese Unexamined Patent Publication No. 2011-24345 (Patent Document 5) discloses a switching regulator that switching-drives the output power transistor in a PWM manner at heavy load and in a PFM manner at light load. However, since the operating frequency is unstable in the PFM manner, the switching regulator operating in the PFM manner might become a noise source to other circuits including the microcontroller.
As another solution to the problem that the efficiency of the PWM switching regulator decreases at light load, Patent Document 1 discloses a PWM switching regulator that includes an oscillation circuit whose oscillation frequency varies in accordance with the condition of the load. An error amplifier amplifies a difference voltage between a voltage obtained by dividing an output voltage of the switching regulator and a first reference voltage, and outputs the amplified voltage. The output of the error amplifier varies in accordance with a load current. The oscillation circuit outputs a triangular wave. A PWM comparator outputs a signal based on a comparison between the triangular wave and the output of the error amplifier. The on/off operation of a switch element in the switching regulator is performed based on the output signal of the PWM comparator. A comparator determines whether the output voltage of the error amplifier is higher or lower than a second reference voltage. The oscillation circuit varies the oscillation frequency in accordance with the determination result. When the load becomes light, that is, the load current value becomes small, the oscillation frequency of the oscillation circuit is lowered, thereby improving the efficiency at light load.
According to the invention disclosed in Patent Document 1, the oscillation frequency is varied based on the output of the error amplifier. Accordingly, the present inventors have found the following problems. First, there is a problem that the error amplifier has a slow response to a change in the condition of the load, which lengthens the time from a change in the condition of the load to a change in the oscillation frequency of the oscillation circuit. The reason is that as disclosed in Japanese Unexamined Patent Publication No. 2007-236051 (Patent Document 6), the transfer characteristic of the switching regulator has a time delay due to a rectification smoothing operation through a switching operation and an inductor, and to ensure the stability of control, the response speed of the control circuit cannot be increased. Second, there is a problem that an efficiency enhancing effect is not obtained as a result when the relationship between the load current and a duty ratio changes. The reason is that the output voltage of the error amplifier corresponds to the duty ratio, and whether the load is light or heavy is determined based on the output voltage of the error amplifier; therefore, the condition of the load is monitored in an indirect manner, and the second reference voltage as a determination reference value is determined based also on the indirect manner. Third, there is a problem that since the second reference voltage as the determination reference value needs to be determined at the time of circuit design, it is difficult to adjust the reference value after the switching regulator along with the microcontroller as the load is incorporated into the system. Accordingly, the efficiency enhancing effect might not be obtained as a result when the duty ratio which is a threshold value for distinguishing between light and heavy loads changes depending on manufacturing variations or use conditions.